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7.
But P. aeruginosa can aggregate even in
liquid
Alhede et al, PLOS ONE (2011)
Aggregates < 105
cells in stationary phase
cultures of P. aeruginosa
Also seen in vivo at sites of chronic infection
Lung tissue from cystic fibrosis
patient, courtesy of K. Kragh
How does the picture of biofilm
development change if growth
starts from preformed
aggregates?
Our approach:
Combine flow cell microscopy with computer simulations

8.
Experiments:
Track biofilm growth in a flow cell
and Kasper Kragh
Chamber is exposed to slow nutrient flow
3D images of biofilm as it grows
Can image over several days

9.
Bacteria are represented as spherical particles
Food is represented as a continuum concentration field
Bacteria consume food, grow and divide
Bacteria push each other out of the way
Food diffuses and is consumed by bacteria
Computer simulations:
Model growth of individual cells with iDynoMics

13.
Simulations: what is the fate of an initial aggregate?
Aggregate has a strong effect on final biofilm structure
This seems to be due to competition for nutrients
Gavin Melaugh
Edinburgh

14.
Cells in aggregate can outcompete single cells at high competition
Average
progeny from
aggregate /
average
progeny from
single cell
Density of surrounding cells (per micron)
Cells in aggregate
outcompete single cells
Simulations:
fate of aggregate depends on
competition from surrounding cells

15.
Key mechanism is competition for nutrients
But cells at top of aggregate
have better access to
nutrients
-> being in an aggregate can
be advantageous at high
competition
Cells in aggregate have a
fitness cost because nutrients
are limited in centre
-> being in an aggregate is
disadvantageous at low
competition

16.
Experimental results:
Fate of aggregate depends on level of
competition
Aggregate can outcompete surrounding cells but only at high competition
Medium competition
Cells in
aggregate
Single
cells
Cells in
aggregate
Single
cells
High competitionLow competition
Single
cells
Cells in
aggregate
Single cells grow faster Cells in aggregate grow
faster

18.
Conclusions
Bacterial biofilms are a beautiful example of multicellular self-assembly
Pre-formed aggregates can change our picture of biofilm development
Stickiness caused by polymer is crucial in aggregate formation
Density-dependent potential can be a way to simulate polymer production
Ongoing questions
What are the pathways to aggregate formation?
Are aggregates a first step in evolution of multicellularity?